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Dark matter that talks to itself could explain galaxy mystery

DARK matter might talk to itself. The mysterious substance that outweighs all visible matter in the cosmos might be best explained if it’s able to interact with itself via an invisible force.

Take a look at any image of a galaxy and you will see that the centre is the brightest. With so much light – and therefore mass – concentrated there, astronomers expected central objects to orbit faster than those on the outer rim.

But in the early 20th century, astronomers were surprised to find that galaxies’ outer stars appear to move about as fast as their inner stars, suggesting that there is more matter that doesn’t meet the eye. The name given to the invisible stuff is dark matter, and the standard paradigm suggests it is composed of weakly interacting massive particles, or WIMPs.

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Now new research on galactic rotation curves – graphs showing the orbital speeds of stars versus their distance from the centre of the galaxy – suggests the story might not be so simple.

Not all rotation curves look alike – before they reach that characteristic plateau, some rise gradually, and others rise rapidly. But WIMP models struggle to explain this. Also, there has been no direct evidence of WIMPs, despite decades of searching. So Ayuki Kamada at the University of California, Riverside, and his colleagues set about finding an alternative.

The team looked at 30 galaxies with strange rotation curves, and found that they could better explain them using a different type of dark matter: the self-interacting sort. These particles do something similar to how ordinary matter particles, like protons, interact with one another via the electromagnetic force.

“It’s a very minimal modification,” says Manoj Kaplinghat at the University of California, Irvine. “But it’s amazing how well it actually fits. You don’t have to bend over backwards.”

When galaxies form, cold dark matter falls to the centre and hot dark matter flows toward the outer edges. But if dark matter is allowed to interact with itself, then the particles will exchange energy and end up at the same temperature, just like the air molecules in a room. In some cases, the cool dark matter particles in the centre will grow hotter and flow toward the outer edges, building a centre less dominated by dark matter – explaining the rotation curves that rise gradually (arxiv.org/abs/1611.02716).

Stacy McGaugh at Case Western Reserve University in Ohio is a critic of the standard dark matter paradigm, so he thinks all alternatives are worth exploring. However, adding new unseen forces to unseen particles complicates the picture unnecessarily, he says.

“It’s what the philosophers of science would call an auxiliary hypothesis on top of an auxiliary hypothesis,” he says. “It’s already ad hoc and we’re adding more.”